I just cancelled my Prime subscription this month. I did it online, and other than having to click "yes I really want to cancel" on three separate confirmation screens, it was pretty frictionless. My subscription was about to renew so there was no issue of a refund, I just cancelled the renewal.
Navigating amazon customer service itself has become a learning experience. Typically the fastest way when getting an annoying customer service agent who “just doesn’t get it” is to request a transfer to amazon USA customer service. You can also request speaking to a supervisot and continue escalating. Sometimes calls end up being 30-45 minutes regardless of method, since they do not always make transferring to amazon USA easy (and then once you get to amazon USA you play the same game over again).
It’s certainly nowhere as good as my original post made it sound, but it’s still better than most online shops.